Semi-persistent scheduling has been considered for Voice over IP in the 3 GPP Long Term Evolution. With semi-persistent scheduling, after an initial resource assignment, the same resource is used on an ongoing basis for a given wireless terminal. Between periods of active speech, so-called talk-spurt states, there is a silent period. A concern that has been expressed is how to efficiently handle the silent period between speech talk-spurts while using semi-persistent scheduling. In persistent scheduling, a resource is pre-allocated for a wireless terminal. During a silent period, if no indication that the resource will not be used by the wireless terminal is signalled, the resource will not be available to be reallocated for use by another wireless terminal, since the assumption must be made that the resource is needed. For very simple persistent scheduling without any enhancements, this means that considerable uplink capacity is lost.
For semi-persistent scheduling, in order to free up the resource during periods of silence state, the resource needs to be re-configured between periods of activity (talk-spurt state periods) and periods of silence state (silence state period) and vice versa. During the silent period, Silence Descriptor (SID) frames may be transmitted, for example to satisfy application layer constraints.
State transition includes the following two cases: transition from talk-spurt state to silence state and the transition from silence state to talk-spurt state. Note that the state transitions are a periodic in the time domain. In one simple way to implement these transitions, whenever a silence state period is detected by the wireless terminal, the wireless terminal sends resource-release signalling to the network to free the pre-allocated resource through the random access channel (RACH) and whenever a talk-spurt is coming, the user will send an explicit request through the existing RACH resource request mechanism for the re-establishment of the semi-persistent resource for transmission of the talk-spurt state. Further, for the SID transmission during silence state, an explicit request is used to ask for the resource, again through the existing RACH resource request mechanism.